As if Idaho’s Snake River isn’t facing enough environmental challenges, a new study now shows how mercury is accumulating in the river’s prized smallmouth bass fishery thanks to an ecological cocktail exacerbated by warming reservoirs and decaying algae.

The study, authored by a team led by USGS ecologist James Willacker, shows how stratification in Snake River reservoirs results in the creation of methylmercury in the lakes’ deeper waters. This mercury then flows out of the bottom-release reservoirs into the river and accumulates in the flesh of top-tier predators. In the Snake River between American Falls Dam and Hell’s Canyon Dam, smallmouth bass have become an apex fishy predator. Salmon and steelhead are no longer present in this portion of the Snake River — the farthest downstream dam in the study, Hell’s Canyon Dam, does not allow migrating fish to pass upstream.

“Impoundments, or dams, are one of the most common man-made changes to river systems,” Willacker said in a news release. “The reservoirs resulting from these dams along the Snake River can affect mercury cycling. We wanted to find out the degree to which fish mercury levels differed across reservoirs and their upstream and downstream environments, and how that translated into potential exposure and health risk to fish, wildlife and humans.” 

During the warmer months, reservoirs along Idaho’s Snake River “stratify” into different layers. The top layer of water, of course, is the warmest, and that’s where summer algae blooms take place. The entire Snake River corridor is agricultural land — farmers grow everything from sugar beets and potatoes to corn and even wine grapes along the river. Nutrients, like nitrogen and phosphorus, from the ag fields eventually end up in the river, fueling the algal blooms.

In time, the algae dies off, and settles in deeper, cooler waters, where it decomposes. The decomposition robs the surrounding water of oxygen creating “anoxic” conditions, which are ideal for the formation of methylmercury. Then, as the study notes, the methylmercury-laden water is discharged into the river and the riverine environment. Every time a smallmouth bass eats an aquatic insect, a minnow or a crayfish from the river, it “uploads” the mercury into its system.

And, of course, the same holds true for fish that live in the stratified reservoirs — they, too, are showing elevated signs of mercury in their systems, and, for the most part, the lake-dwelling fish are displaying the toxic contamination even more acutely. The silver lining? Smallmouth bass living in the river above each of the tested reservoirs displayed the least amount of mercury contamination.

Nevertheless, the study reveals that even the bass least contaminated by mercury approach the EPA recommended limit for the toxin if they are to be eaten by humans. And, the least-contaminated fish studied were 1-year-old bass — likely too small to be targeted by anglers.

The EPA’s human-health standard for mercury in smallmouth bass is .3 micrograms per kilogram of wet weight. While younger bass and bass that approached reproductive age tested for the study showed some mercury contamination, bass deemed of “harvestable” size often exceeded the EPA’s allowable limit, sometimes significantly. Some harvestable bass caught and tested from stratified reservoirs and those caught and tested below dams showed mercury levels as high as twice the EPA’s human-health criterion.

“Within both reservoir and tailrace habitats, stratification regimes substantially influenced the exceedance of the EPA (methylmercury) criterion,” the study reads. “Over 75 percent of the harvestable (305 mm) bass exceeded the criterion in inconsistently stratifying reservoirs and tailraces below inconsistently or consistently stratifying reservoirs, compared to only 6 and 17 percent of harvestable fish from unstratified reservoirs and associated tailraces.”

In all, the study’s authors tested smallmouth bass for mercury contamination in, above and below 13 dams on the Snake River in Idaho. Throughout the study, harvestable fish tested from stratified reservoirs proved the most contaminated, followed by harvestable fish caught within 25 kilometers below a dam on the river.

“The results underscore the influence of reservoirs and their biogeochemical conditions as potential drivers of mercury exposure risk,” said co-author Collin Eagles-Smith, USGS supervisory research ecologist. “Resource managers can take these factors into account as they evaluate current advisories in a manner that balances harvest opportunities with consumption risk.” 

The study adds just another layer of complexity to the challenges facing a deeply troubled river system like the Snake. What’s more, if mercury contamination is significant in smallmouth bass, it’s likely just as acute, if not more so, in other fish in the river, like sturgeon, carp and even hatchery trout that seemingly thrive in the cold waters directly below the dams.